The present invention relates to a method and device for switching between different types of output signals of a position measuring system wherein at least two different types of output signals can be generated by the position measuring system which are transmitted via signal transmission lines to an evaluating unit and wherein switching between the different types of output signals takes place by of one or several switching signals transmitted by the evaluating unit or a supply line.
One known type of incremental position measuring system is designed so that analog scanning signals generated by scanning a periodic scale graduation are transmitted to an evaluating unit connected downstream. Other types of incremental position measuring systems are known where the digitalization of analog scanning signals takes place in the position measuring system itself. The digitized signals are then transmitted in rectangular form to the evaluating unit connected downstream. The digitized signals can be TTL, or HTL signals, i.e. the voltage levels of these signals are different. The selected type of signal transmission and therefore of the corresponding position measuring system respectively are a function of the requirements of the evaluating unit.
In connection with position measurement systems so far known, only a transmission of analog or digitized output signals of a defined type is possible, i.e. the transmission of only one of the various types of output signals mentioned. However, it would be advantageous if several types of output signals could be selectively available to the evaluating unit in an incremental position measuring system.
For example, in the case of the transmission of digitized output signals in TTL or HTL form only, difficulties sometimes arise when the emitted rectangular signals are to be checked with respect to the quality of the analog scanning signals. If, for example, problems occur on the scanning unit side of such a position measuring system during the measuring operation, this manifests itself in the evaluating unit only by the loss of transmitted signals. A more thorough error analysis is only possible directly at the scanning unit.
The requirement for monitoring the generated analog scanning signals furthermore arises in the course of an adjustment of such a position measuring system, or respectively in the course of a possibly required alignment of such a measuring system. If only rectangular signals are available at the output side of the scanning unit, it is necessary to detect the analog scanning signals within the scanning unit for this purpose. This again requires to open the scanning unit to allow access to the analog scanning signals.
For these reasons it has been proposed, for example, in Japanese Laid-Open Application JP 08-029197, to provide the opportunity of selectively setting a definite type of output signal in an incremental position measuring system. In this case the transmission of the analog signals is also provided along with the transmission of rectangular signals. Switching between the two different types of output signals is performed by a switching signal provided by the evaluating unit. This switching signal is transmitted on a separate line coupling the scanning unit and the evaluating unit. However, a disadvantage of this type of a position measuring system is the requirement of a separate line between the position measuring system and the evaluating unit for transmitting the switching signals.
It is thus desirable to provide a position measuring system in which the scanning unit may selectively output different types of scanning signals, i.e., analog and digital scanning signals. It is also desirable to provide a position measuring system that does not require additional transmission lines in order to provide the selectivity of different types of output signals.
It is therefore an object of the present invention to provide a method and device for switching between different types of signals output from a scanning device used in a position measuring system without requiring a separate or additional connecting lines between the scanning unit and the evaluating unit for transmitting the switching signal.
According to a preferred embodiment of the present invention, an already present supply line used to connect the position measuring system, i.e., scanning unit, and the evaluating unit is used for transmitting a switching signal. Thus, no separate or additional connecting line is required. The expenditure to implement the present invention in newly constructed position measuring systems or in refitting existing position measuring systems is therefore reduced.
In a preferred embodiment of the present invention, a switching signal of a defined length and/or a defined frequency is modulated upon the supply line and the modulation is identified on the position measuring system side. In this preferred embodiment different switching signals are available for the different types of output signals. Switching between the different types of output signals can take place using such a switching signal.
Alternatively it is also possible to use different levels of d.c. voltage distribution as a switching signal when it is desired to switch between different levels of rectangular output signals, for example, between HTL and TTL signals. Depending on the determined or detected level of the applied supply voltage as a switching signal, output signals in rectangular form with different signal levels are transmitted to the evaluating unit. In such a preferred embodiment it is only necessary to design the position measuring system so that it can be operated at different supply voltages.
A further advantage of the present invention is that the number of different structural components of position measuring systems can be reduced since it is possible to select the required type of output signal from the same position measuring system.
The device and method according to the present invention can of course be employed in connection with the most diverse types of measuring systems.
Further advantages as well as details of the device and method according to the present invention ensue from the following description of the preferred embodiments.